1. Field of the Invention
This invention relates generally to speed and directional control of a DC motor, and more specifically is directed to bidirectional control of a DC motor with a single speed sensor.
2. Description of the Prior Art
The prior art contains examples of DC motor control circuits which permits constant torque to be developed by the motor regardless of the rotational angle of the motor shaft. For example, U.S. Pat. No. 3,383,574 and No. 3,517,289 teach the use of Hall effect elements which control the current fed to motor armature windings in sinusoidal relationship to the rotor angular portion to achieve such constant torque. The noted patents also teach that torque, and consequently motor rotation, in either direction is achievable by controlling the direction of current through the Hall effect elements.
U.S. Pat. No. 3,839,661 teaches a DC motor using Hall effect elements and a speed control circuit which controls the Hall effect elements in proportion to the speed of the rotor.
A convenient source of a speed signal which can be used to generate speed control signals is a sensor or frequency generator which generates a frequency proportional to the speed of the rotor. A sensor of such type may employ magnetic pole pieces arranged with alternating north-south polarities and being uniformly spaced about a disc which rotates with the rotor. One or more pickup heads fixedly disposed within the influence of the pole pieces has induced therein an alternating signal with a frequency proportional to the rotational speed of the disc. A frequency-to-voltage converter is provided to generate an analog voltage having a linear inverse relationship to the frequency output of the pickup head over a substantial range of frequencies. When the voltage output of the frequency-to-voltage converter is employed to control the current to the Hall effect elements and the resulting torque of the motor, a substantially constant speed of the motor can be achieved.
The output voltage of the frequency-to-voltage converter of the prior art becomes zero at and above a certain cutoff input frequency. If the speed of the motor overshoots the desired speed and exceeds the speed which generates the cutoff input frequency due to, for example, excessive acceleration during start-up or during speed change, the zero output of the frequency-to-voltage converter, if directly used to control the current to the Hall effect elements results in zero current to the Hall effect elements and zero torque from the motor. Thus, the motor merely coasts until turntable load, friction and windage reduce its speed sufficiently to return the input frequency to the frequency-to-voltage converter to the range of the linear relation. When in this linear range, the motor speed is controlled to the desired speed.
U.S. Pat. No. 4,135,120, issued Jan. 16, 1979 and having a common assignee with the present application, employs an offset voltage which is combined with the output voltage of the frequency-to-voltage converter to reverse the sense of the current through the Hall effect elements before the output voltage of the frequency-to-voltage converter reaches zero. Thus, the motor torque is actually reversed in the frequency region beyond and just less than the cutoff frequency. This reversed torque rapidly decreases the motor speed and forces the motor to rotate in the proper forward control direction.
In the system described in U.S. Pat. No. 4,135,120, reversal of inputs can intentionally or unintentionally cause a reverse of the direction of motor torque. A problem arises with the described device in the reversed-torque speed region near and above the cutoff frequency. Assuming that the motor is arranged for rotation in an arbitrarily chosen positive direction but is, for example, manually rotated in the negative or reverse direction at a speed high enough to generate the cutoff frequency, then the offset voltage is effective, upon the attainment of the cutoff frequency in the negative direction, to produce a negative torque. Since the motor is already rotating in the negative direction, the applied negative torque tends to further increase its speed in the negative direction. In this runaway condition, the speed can continue to increase until the equipment is damaged.
Speed sensors of the type described are bidirectional, that is, their outputs contain no indication of the direction of rotation. One method of avoiding runaway involves sensing the actual direction of rotation of the motor. An arrangement for sensing the rotational direction employs two pickup heads disposed within the influence of the pole pieces so that a predetermined phase difference, for example, of 90 degrees, exists between their respective speed signals, and the phase difference permits discrimination of the direction of rotation. However, such arrangement requires an additional or second pickup head and an associated amplifier as well as a circuit for interpreting the phase difference between the two speed signals and other circuits for preventing driving in the improper direction when the latter is discriminated. Such additional equipment increases the cost and complexity of the apparatus.